"We think that since most children are encountering viruses for the first time, their immune systems aren't able to kill them as quickly as when they encounter them again," says Carol J. Baker, M.D., a professor of pediatrics at Baylor College of Medicine in Houston.

What's more, because kids aren't overly concerned about having a runny nose, the virus tends to end up on their hands, clothing, and toys — where it can live for 30 minutes. When another child touches an infected toy and then rubs her nose or eyes, she can catch the cold.

However, having lots of sniffles early in life may protect kids later on. Researchers have found that children who develop frequent colds in preschool catch fewer colds during their school years — presumably because their immune systems have learned to recognize and fight off the bugs. And a German study found that babies who have more than one cold before their first birthday are less likely to develop asthma by age 7.